Airport terminal buildings are some of the largest and most populated multifunctional buildings in the world. Terminal 1 at Lester B. Pearson is Canada's largest airport, the hub of over 400,000 flights per year supporting over 30 million passengers - exceeding the predicted capacities. The terminal has an area of approximately four million square feet, accommodating retail shops and 80 gates. There are many different uses and occupancies within Terminal 1 including: public assembly, private office space, back-of-house facility service space (woodshops, metal shops, repair facilities, cart storage and charging areas, secure and non-secure areas, RCMP, CATSA, and police detention areas.

Each terminal building presents its own unique challenges but there are common elements affecting the mechanical design that are encountered on all projects. The mechanical systems are selected to suit a variety of zones and conditions and must be flexible enough to accommodate large swings in population: both relatively stationary (at check-in counters, security checkpoints, baggage claim and gates) and in continuous circulation (such as corridors). Such occupancy swings, which may occur hourly, daily and seasonally, impacts the size of air handling units as well as thermostatic zone control.